Unless you intend to do a bunch of light air racing, 5 oz cloth sounds absurdly light for a mainsail for a boat like the Tanzer. A J-22 racing mainsail is typically around 6 oz and these sails are so light that they have very short useful lifespans.
I would also not necessarily suggest going with an offshore spec for a Tanzer 7.5. Unless you spend a lot of time sailing in heavy conditions you do not need the added weight that is assoicated with an offshore sail and that added the weight would really reduce the lower end of the useful wind range of a mainsail on a 24 foot boat without actually increasing the useful life of the sail.
I would also strongly suggest that you go to full length battens.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies