Best place to put feliway diffuser?

I just got a feliway diffuser to relax Luna more. She sometimes pees right in front of the litterbox and I think it's from stress (they also say that feliway is good to eliminate couch scratching) even though she doesn't act stressed in any other way. My cousin brings her kitte, Salem, over to play with Shermie, kinda like a play date, and while we have spent time introducing Luna to the new kitten, she seems to do the pee-thing when he's over. (Although, Salem leaves Luna alone, unlike Sherm who constantly tries to play with her). She is still social when he's over - not hiding, but hissed a couple times.

So, I have a 2 floor condo w/ one litter box in the basement and one in the main floor laundry room. She has peed outside of both, though she does the laundry room more. Should I put the diffuser in the room w/ the litter box or the main living room area? Would it be helpful (next time i get paid) to get a 2nd one to put in the basement?

Any advice would be helpful.

Also, there seem to be random times when she'll do the pee-thing after a long time of not and for no reason that I can tell..

(by the way, she's been treated for UTIs and all that stuff by the vet and it's all good.)

I have a 1,100 sq. ft. apartment and have only one diffuser in the living room, where they spend the most time. Could you put one on each level near the litter boxes?

You should try the spray too. In my experience it works better than the diffusers but I think it depends on the feline.