A few factors play in when it comes to lewdness with panties.
First off, the trait "Squirter" a girl can have, which increase her cum by 2.
It's possible to add traits with shift+o

Can't recall the command, but search function should locate it?
Secondly a girl can wear "dirty" panties if you don't steal them and then you can continue to dirty them

Not sure how your own cum in panties affect panty lewdness, but I know their price value drop by it for some reason?
Not knowing what mods you've installed.. one makes it possible to steal from the girls locker in bathroom, sometimes very dirty panties can be found in there.
Being able to cum a lot also helps.. as in you can make a girl cum in her panties a lot more times before you run out of own climax's... Blue Pill and Medical upgrades
With all that said, it is a pain sometimes to get the right kind of panties with the correct ammount of lewdness, especially if a short sponsor time deadline for it. Therefor I personally use the cheat for adding clothing.
Without cheats panty hunting becomes a chore IMO
You can also ask the sponsor for another type of request, this will lower the relation points with the sponsor, but forfilling the other request instead increases it, so you loose less points.. might even break even?