Players have come to a consensus that 90/90/90 is the minimum you'd want to put for a chance at a gold tank. If you wanted a better chance at gold tanks, it would be 130/130/130 - 142/142/142 or higher. If you just want purples, go with 30/30/30.
It's usually advised to go for 142/142/142+ only when you get the BWMG to lv2, because it will then unlock N5 tanks (don't quote me on this).
Put an attire of the same type of the tank you want. It might help~
It's not a joke...It used to be a really nice spreadsheet, and i've posted there my results before, but since it was open to the public, some troll decided to replace everything people have put in with porn...the lenghts people go out of their way just to ruin the life of someone else amazes me everytime.
I was following the steps on building equipments on the Panzer Waltz website (https://panzerwaltz.gamepedia.com/BWMG_Depot) as I was trying to get some Active Detectors. It gave me a 61 / 65 / 41 and I used Pz III Prototype's attire and, I got an Alecto Mk I. Wait time was a bit under 6 hours and, running the same formula, I got a Tetrarch with wait time of just about a hour.