tileboard tub surround transition caulking

I got an odd situation with the transition between our one piece tub surround and the tileboard I installed above it. The tilebard was installed over drywall and is completely out of the shower spray area but will be exposed to minor splash and humidity, therefor it is imperative to seal the edges well with caulk. This is not a problem except for about a 2 foot section at the middle of the back wall where, because of the surround's profile it dips to about 1/16" lower than the level of the tileboard which will make calking almost a reverse of the normal corner caulking. Normal tooling of the caulk at this point will only barely cover the uncoated edge of the tileboard. Anyone have any ideas! It's almost like I need to make a convex bead of caulk at this section to achieve at least a 1/8" overlap of the board edge. Thanks for any ideas.