Best way? Keep it simple! (Don't overthink it.
Your 'beam' antenna doesn't require pinpoint laser accuracy to sniper shoot your repeater of interest unless you're that way inclined. Being in the ball park will generally be enough or otherwise known as the general direction.
The first app that may be of help is "Repeaterbook" to tell you what repeaters are in your area, their distance and helpfully also the compass heading from you. So long as you know where you are and where the repeater is you can point your antenna in the right direction using a simple compass even such as that built into mobile phones nowadays.
With a loft installation and fixed antenna it might be worth taking your handheld radio up with you so that you can test repeater access in situ. Much better than climbing up and down the flipping loft tweaking the antenna each time! Then do your final coax feed. If using an antenna rotator just make sure you set the 'North' setting correctly, (using your compass), then figure it all out in the shack with which repeater is at what heading and make notes.
My own DIY Yagi-Uda simply sits on a stand in my shack up near ceiling height and I've got quite adept at swinging it around by hand for whichever repeater I want to access. (Just a six element, a ten element may require more swing room!!) Some would say that's sheer madness but then I don't want a dirty big white stick colinear hanging off the chimney stack waggling in the wind. (Or long lengths of expensive coax feeding it!) With that setup I can access seven UHF repeaters in my 'local' area, (several counties worth!), and it all works quite well.
Be aware that even if you point your antenna in the right direction it doesn't guarantee you hitting that repeater. Your local geography may well affect you and in my instance I have a dirty big incline North of me as well as huge landfill with massive steel box warehousing to the West. To talk to my old Ham friend 20 odd miles to my West I have to fire to a repeater the next county down SSW of me through a gap in the hills that he can pick up the other side in his county. (Three county span that way but works!)
There are coverage maps available for repeaters but despite best efforts they're not all that accurate, (I can hit repeaters that such maps show I can't and not access repeaters where the maps show I can?!?!) There's also some 'special' radio coverage modelling software around but after playing with several of them they're not all that accurate either.
Here's hoping you find a repeater of interest, it can be as barren as the Sahara out there with little life on some repeaters. A lot might have regular 'nets' so worth scouring club sites for when that usually occurs. Do be aware that some run their little nets with the "this is a local net for local people" sort of attitude and such small groups can be festering grounds for small minded ideologies. Surprising that some can publicly broadcast racist or misogynistic messages ashamedly across the airwaves or berating other groups of society. There but for the grace of God go I?
If you find a good repeater with a good crowd accessing it support it with all your heart by using it and/or donating to it or even consider membership of the repeater group. But also don't feel pressured to, repeaters do not operate under a 'toll' charge system.
Anyway, apologies for the waffle but hope it helps somewhat. Most of all Simon have fun with it all and keep the excitement as there's no better feeling than some radio success to put a smile on your face. Even if someone else is less than impressed before berating your 'Chinese' radio.
All the best,