Different SSRIs will probably have different effects on your trip, but....
Yes, SSRIs in general will "take the edge off" LSD for most subjects; most report this as a positive change, making it easier to handle the experience and allowing one to be less emotionally volatile...but for some, it interacts poorly with the LSD and produces a range of negative effects.
As with any psychedelic -- particularly when compounded by the effects of another class of substances not entirely understood -- it will effect everyone differently. However, in general you should expect decreased visuals, greater psychological stability and calm, and other subtle changes to the "feel" of the trip.
I don't know that an SSRI will actually cap your trip at a certain level....if you take more, you should be able to scale the experience right up to a tornadic freak-out with significant ER potential, just like always. It will just take more LSD to get you there.
Thusly, don't go hog wild. Maybe just go with your usual dose, with maybe an extra 25% or so available if you want it after you come up.
Or, you might consider the reason you're on an SSRI in the first place, and whether LSD will really help you rather than set you back. Know that a great many of us who've had "acid phases" wish we'd been more cautious and respectful -- despite the fact that we generally felt like we were being both at the time. Acid can do great things for a person, but not if they've become too uncomfortable with hyperspace that they no longer want to go there.
In any case, good luck and godspeed....
