what i can say is that there is a MUST to have premiun in order to burn experts. It WILL give you better chance.
the other factor to consider is the "event" 200% conversion rate bonus, gived by SDE on certain events (the past one on weekends was one, anotherone is the Christmas event)... there used to be only few of this events a couple of years back, but since the game is slowly dying this events come every couple of months.
To "boost" a sailor from 100 to 140 vets your sailor can eat less than 100 to about 400 experts. (with prem on 200% rate event)
if you plan to "boost" an entire (lets say 8 supports for a BB) crew from 100 to 140.. with prem and 200% rate, about 4 will reach 140 with no more than 100 to 150 exp, 3 of them will eat 200 to 300 experts and one pesky sailor will eat 400 experts to reach 140... (thats in my personal experience pretty generalyzed, when you burn a set of sailors a couple will have no problems, another couple will only have minor issues, and there's allways a last pain in the but).
Consider to buy 2000 experts for lvl up crew. and if you are smart you will also buy a 3 day prem (only 40 olives) to burn with high rate
btw... if you have the chance, convert expert to vets from 0 to about 60... then you take it with single vets up to 69... and then you use 2 10 vet pack to take it to 109... its cheaper to use vet pack than burning from 70 to 109.
And Allways (from the moment you reach it) use your sailors BVE, there is no point on reaching 140 vets if you leave the sailor with no experts, it will perform worst and with normal expert win (even using AA) reach 300 expert migh take something like 50 to 100 battles... that's A LOT. If you're going to burn... be shure to have the expert replacement waiting on your item depot