Before I go off on how following the instrutions on this to a tee is super important I have a question.
What's under those tiles? If it's old plaster or sheetrock in any form it's very likly your going to regret not doing this right and removing all of the old tile.
No floor is ever perfectly level, the tub it's self is not going to be perfectly flat and level.
If there's nothing to 100% fully support the tub it's going to flex, flex = cracks in the tub, cracking caulking, can cause the drain fitting to leak.
In an older home it's also all most always advisable to replace any old steel supply of drain lines, and the old control valve.
What's under those tiles? If it's old plaster or sheetrock in any form it's very likly your going to regret not doing this right and removing all of the old tile.
No floor is ever perfectly level, the tub it's self is not going to be perfectly flat and level.
If there's nothing to 100% fully support the tub it's going to flex, flex = cracks in the tub, cracking caulking, can cause the drain fitting to leak.
In an older home it's also all most always advisable to replace any old steel supply of drain lines, and the old control valve.