-5 to +7 so approx 2/12 chance to have one with more than +5% bonus
so you need approx 6 sets of (one gravid, one web) = 6×(3+1.2) = 25M to have a correct web. In reality you gamble like 100M, so approx 24 mutations, and you take those that have enough web strength and bad other stats since you don’t care about the other stats.
You should have around 4 (maybe 2, maybe 6) web with a strength bonus over 5%, which is enough to catapult a 24s align freighter in 2s. actually because of lower evaluation it’s enough for like 26s or so ; and actually you will find like +6% web with crap other stats which is definitely enough.