Several different questions here. Let's try to take them one by one.
but car is easy, just slot it in, wait for the click.
That is because cars are designed differently. If you could put the fuel tank into, say, the panniers of a motorcycle you could do the same thing.
is it best to do it on centre stand?
There is no 'best' or 'worse' rule written down anywhere.
I have been sitting on the bike doing it, (pay at pump)
You can do it that way if you like. You can also do it that way when you pay at the kiosk, too.
now with tank bag, have been doing it on the side stand,
I guess that you are taking the tank bag off?
OK
You can now either leave it on the sidestand and fill it, or, lift it onto it's centre stand.
but as it leans over a fair bit, i don't think i get as much fuel in.
Liquid (fuel in this case) will always be level; it doesn't run uphill, that is for sure.
You are of course assuming that the complete volume inside the tank fills when the bike is fully vertical. I haven't looked but I think (always a dangerous position) that the 'crown' of the tank never fills, there always being an air gap at the top. No doubt there is a cross-section drawing somewhere.
If I am right, then putting the bike at an angle may well make no significant difference. There is a sticker on KTM's (with their fancy two cap fillers) explaining to owners that the lower tank should be filled slightly less.
As to the difference in volume inserted? There is a reasonably easy way to find out.
Drain the tank to empty. Brim it vertical and measure the amount of fuel poured in. Drain the tank again and measure the amount of fuel poured in, with the bike on the side stand. If the numbers are different, subtract the smaller number from the larger one. That figure will be the exact difference. Repeat two or three times to exclude any possible variables.
As you will rarely (I hope) ever ride the bike to completely empty -thereby leaving yourself potentially stranded in the wide open world - the differnce is probably not significant.
How many people re-fuel on the centre stand?
Run a poll for an accurate assessment. Or watch when you fill up. Either way, it makes little to no odds.
Another silly question, do you rely on the back pressure to 'click' the pump, l
Why not, it's what the pressure switch is there for. You may well notice that some are more sensitive than others.
i am always bit nervous of it splashing fuel all over the tank.
That's good. It also stops fuel pouring onto the garage forecourt, an anti-social and wasteful habit. Fuel is expensive, don't waste it.
With the car its just bumper, and don't mind.
l
See answer above. I am sure nobody wants to think you a slovenly, anti-social car driving fuel filler, so try not to do it, please. Besides being wasteful, fuel is very slippery. You run the real risk of one of your fellow bikers (I love that word) slipping and crashing off or over. I am sure you would not want it to happen to you, when you are in your 'biker' mode?
I have been keeping the nozzle at an angle so i can see the fuel going in once it gets near full
Well, that's a good idea. If you put the bike right up on the centre stand, you can lift the nozzle completely clear of the filler hole and run fuel in. This obviously avoids the safety net of the pressure stop valve (and brings with it the possible danger of going into a daydream about filling methods) meaning that fuel can spill. See above.
But, it means you can brim the tank. This ensures you have put the maximum amount of fuel in. One day, that extra bit may well save your life, who knows.
Of course, if you are very short or struggle to lift the bike onto its centre stand, you will always be stuck with either sitting aside it (possibly at a slight lean) or on its side stand. Rendering all the above interesting, but in your case, irrelevant.
H&S WARNING!......Do take care if sitting astride your bike when filling. Your foot may slip (possibly due to the floor being slippery from spilt fuel, left by anti-social car drivers who do not care about spilling fuel down their bumpers.... and onto the ground....see above). If you tumble off it may well hurt, may well damage your bike, spill fuel and you will look like a tit.
One tip, though, if you are not blessed with many inches. You can sometimes stand on the raised plinth of the petrol pump, giving you that extra height advantage.
One question you have not asked: Do you take your helmet and / or gloves off when filling? But, I leave that one to you.
)
will think yer goin' to 'Do One' without paying...........or buy a flip front.
Or a least a way for your to solve the mystery that is bugging you.
