I desperately need advice as to how to fill a large gap between my tile bath wall and the tub. It had been filled with grout over which silicone caulk was applied. It was done sloppily and rotted the backer board. I have repaired it twice, once with regular grout (it cracked and leaked) and on the advice of a Lowe's "tile specialist" with silicone caulk only. The problem is, the space between the tile and tub is 3/4 inch on the long wall. The backer board ends at the bottom of the tiles so the caulk has nothing to adhere to. I have purchased a grout formulated for filling large joints hoping that since it is designed for larger joints it will be less likely to crack. I plan to caulk over the grout in the event it does crack. Help!!!! I don't want to remove anymore grout or caulk!