Alvaro's solution is good, as it most likely will result in better-precise result (and I'd say enough precise one). I just confirm that yes, GLSL has mod and GLSL fmod. Note that you have a bug in the code you wrote, the second line should actually have 'inc' as 1st argument into mod on 2nd line.
Now to the thing I wanted to write ... in case you would need even more precision, you *can* use double-precision floating point numbers on gpu, in OpenGL it's through extension GL_ARB_gpu_shader_fp64, in HLSL I'm sure that in D3D 11.1 (I also think that 11.0 supports them but I'm not sure -> if someone checking this works with D3D, please confirm), it is possible to use them.
In OpenGL you can use glUniform*d, glUniform*dv, glUniformMatrix*dv and also when GL_EXT_direct_state_access is supported, then glProgramUniform*d, glProgramUniform*dv, glProgramUniformMatrix*dv. Your variables are then declared as double/dvec/dmat in shader.
Edited by Vilem Otte, 11 December 2013 - 02:45 AM.