This line:
Costilt = (Xvert*Xos)+(Yvert*Yos)+(Zvert*Zos)/(Modvert*Modmove);
needs to be changed to
Costilt = (Xvert*Xos + Yvert*Yos + Zvert*Zos)/(Modvert*Modmove);
(The first line, with the incorrect formulation, "sort of" worked because Xvert and Yvert are so small).
To get degrees from acos(), use
Tilt = (180.0/M_PI)*acos(Costilt);
Did you calibrate that particular accelerometer to arrive at the offsets and scale factors used in the program?
If not, that needs to be done for each accelerometer individually. See this tutorial: Tutorial: How to calibrate a compass (and accelerometer) with Arduino | Underwater Arduino Data Loggers