Have a look at my answer to https://physics.stackexchange.com/questions/29883. To a good approximation the transmission falls exponentially with thickness. Just work out what tranmission you need, e.g. if you want 80% reflectance choose 20% transmission, and work out the film thickness accordingly.

You can find the optical constants for titanium at http://refractiveindex.info/?group=METALS&material=Titanium