Many years ago, before such things as a flash meter.
Guide numbers were determined by shooting a roll of film. To start, make up some cards with f values on them.
With an indicated GN of 80 number the cards f5.6, f5.6+1/2(6.3), f8, f8+1/2 and f11. Make one exposure at each
setting, have the film developed and the best exposure is YOUR GN with that flash unit. Same thing for near distance, 1 stop
under, 1/2 stop under, recommended, and + 1/2 and 1 stop over.
That'll set you up.
Make the numbers dark enough to read in the print, using a ball point is fruitless.