What's the right way to mix and use boric acid spray for fleas?

My fleas were begging for it, so I let them have it.. sort of

After an hour of trying to figure out why my sprayer won’t spray properly I have realized boric acid is not soluble in water. Or is it ?

As i have read over the interwebs, boric acid = borax + salt in a 5:4 mix ratio. There were crystals in the water + boric acid mix (about 200g per 4L) which would clog my sprayers. Would I be guessing correctly those insoluble crystals were the salt ?

I took out the crystals, put the solution in my sprayer and sprayed all over the place. Upon drying a noticeable film of something got laid out where I have done the spraying. Is that the borax ?

And for future referents, should we buy just borax instead of boric acid if we wanted to dissolve it in water ? What ratio borax : water if so ?