For a 1-7 sec exposure I DOUBLE the indicated time on the meter.
For 8-59 seconds I QUADRUPLE it.
For 1 min plus I multiply by EIGHT (from a table published in Photo Art a few years ago). This has served me well for several years and I'm very happy with the negs-a touch dense sometimes but if it's not on the neg you can't print it can you ? I've generally used FP4/HP5 or Fortepan in 4x5 devved in PMK or a 2 bath.