Ok, what length steerer do I need?
Have frame no headset fitted, need to buy some used forks.
Is there an average length I need to add the head tube length ie50mm?
Don’t want to buy forks and the find steerer to short!
Head tube measures 125mm
Thanks

Why not get the headset, stem etc then measure it? If you add on ~30mm for spacers that’ll get you in the ballpark.

I think you’ll definitely need to narrow things down a bit. There’s a big range in headset stack heights now, depending on what sort of headset your frame needs (of course what headset type you need can depend on the fork steerer type too). Get some sort of handle on that, and what stem you want to run. Add them together, add a bit, then cross your fingers you’ve worked it out OK

The very lowest stem height you’ll get is 30-35mm, normal stack closer to 40mm. Headsets likely to be in the range of 20-35mm. Plus a few spacers for fit/luck. So you’re looking more at steerer plus 80mm, maybe more.

I’d allow about 50mm for the headset (assuming its not integrated / recessed), 40mm for the stem, plus any spacers you want to use, then add those to the length of the head tube.
Its probably more than you’ll need, but better to over-estimate. You cant make a steerer longer, but you can always shorten it!