in general, can you use the large to large sprocket plus one link chain sizing method on a bike with a single chainring and multiple rear sprockets? Or do you need extra length due to the fact you will actually use the large/large sprockets?
thanks

Big-big plus 1" should work for anything. However since you expect to spend real time in big-big, not just need it available as a disaster prevention, a few extra chain links won't hurt. Just be sure your rear derailleur can wrap up enough chain to stay under a bit of tension in the smallest cog.

thanks for your reply

As long as the derrailleur doesn't seem like it's G string tight with the big-big +1 then I'd go with it. If it seems strained add a second link pair and call it good. there's really no need to use more than that I would think.

This same method is how I did my own 1x8 errand bike up and it's worked out fine for years now.

I would be hesitant to add too many extra links to a 1X8/9/10, unless you have a really excellent derailment prevention system using a too long chain is just asking for a thrown chain.

It's actually an old BSA with a 1x4. Does more/less sprockets make a difference?
thanks