Engine break in without a tachometer

Hey all, apologies if this has already been posted, I searched but couldn’t find anything.

I’m getting ready to fire up my 318LA after rebuilding it, and my car doesn’t have a tach. I have hydraulic flat tappet lifters. I do have an aftermarket tach, but I really don’t want to fuss with installing it right now. How crucial is it that I stick to 2500 RPM during the break in period? Can I apply some light throttle and stick to it for the 20 minutes and be good?