As a fellow ISP shooter I thought I'd put my tuppence worth in.
Firstly, 25 yards isn't enough for testing. I've seen pellets group really well at 25 but rubbish past 45 so you need to look at the full 55 yards for your initial sift, 25 yards just establishes your technique is OK. You should test at all distance but start at 55 and work back. Secondly, I can feel this becoming perceived as a slightly negative response but please take it in good spirit, if you're struggling with wobble at 25 yards then no amount of pellet testing is going to make a tangible difference to your score card. Work on getting steady, even a pellet that groups on a pin head wont fundamentally change your score if your underlying technique isn't solid.
That aside, aim to get groups the size of a 5p piece with a few going just outside. Don't worry about where they land in relation to the crosshair whilst testing just look at the group size. I purposefully under dial when I'm testing pellets to get a group about 20mm below my aimpoint so that whatever I'm aiming at doesn't get obliterated. Try them dry first and then lubed. Lube is for lubing the barrel, the pellet is just the carrier, so if you swap back to unlubed you'll need to thoroughly clean the barrel as it will still be lubed for some time. If your ISP is anything like mine it only takes a few shots to get the barrel working after a thorough clean so you might want to clean between each batch.
If you're shooting outside there will be some wind so don't worry about sideways drift. You're looking for them to create a horizontal line of holes but obviously if it's really windy then don't bother. It does need to be as still as is possible. Don't give any windage when testing, just aim at the marker and allow the wind to take the pellet. Aiming off to compensate for the wind doesn't help, in fact it tells you less about the pellet.
My ISP isn't particularly pellet fussy, it'll shoot most JSB exacts pretty well in 4.51, 52 and 53 but some batches are notably better than others. It doesn't like the JSB premiums though, something like 4 out of 5 are spot on and the 5th drops 20mm. Tried 3 different batches and the same story for each. I don't use lube, seems to make things worse but other ISP's like lube so try for yourself.
Don't mess around with the power, keep it between 775 and 785 with 8.44 grain and only use the rear hammer spring adjuster. Moving the trigger block to adjust the hammer travel is a nightmare, Shaun sets them up to be very fast but consistent so just use the rear screw unless you start to run out of travel. Fine tuning the power to suit the pellet is a bit of a myth, loads of people believe in it though so who am I to mock. That said, velocity can have an effect on gyroscopic precession (the major factor in pellet accuracy) as the faster it's going forwards the faster it's spinning due to the mechanical relationship between velocity and spin rate whilst in the barrel, but 5 or 10 fps isn't going to have any significant bearing unless the pellet is on a knife edge in which case it isn't a pellet you'd want to use. There used to be a view that 777fps was the magic number, something to do with barrel harmonics. I shoot at 775 because it gives me decent headroom at the chrono lane and it seems to work fine.
Hope that helps
ISP Airstream MK III No 15 which is a bit poorly at the moment and having a rest
AA Pro Sport + Ginb FT8 stock + Bushnell 4200 6-24x
EV2 MK1 + Leupold Comp 40x
Weihrauch HW100 KT laminate
SBFTC Competition Secretary