Yeah, what I would do is take a pan of solvent without the LSD of known mass, soak a blotter in that and let it drip until it is exactly saturated and wet but not literally swimming, then see what mass of solvent is still left and substract the masses.
Alternatively first weight the blotter and after absorption weight it again to see the additional mass of solvent.
Then take a square or rectangular container that is
just larger than the blotter sheet, put in the solvent containing LSD weighing the saturation amount and smoothly lay the sheet in that.
I would make it just a tiny bit wetter than the sheet can handle, so tiny that evaporation of the solvent while still laying in the container makes the surface look kinda dried in, seems better for uniform diffusion that way. When it's somewhat dried in there also shouldn't be any solvent left when you take out the sheet.
It would be wise to get a perfectly adjusted horizontal table and maybe even lay some kind of inert material on the blotter paper to weigh it down preventing a tilt, that is permeable for evaporation like a gauze but more massive.
After taking the paper out and letting it dry completely I would put it on a perfectly horizontal surface once again just to be sure.
Maybe some preventive measures in the room you do it in so there's little degradation during the process - of course you're just handling liquid but there could be things to avoid.
Dunno, that's just exactly what I would do if I think about it

not saying this is how it is done.
Though what I would do in reality is try and find out how the pro's do it and use that info lol.