Have a look at my answer to Make a semi transparent mirror with copper. 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

Have a look at my answer to Make a semi transparent mirror with copper. 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