We have a 45 gallon pressure tank and are not sure how much air to put in the bladder or bottom of it?

Don't put any in the bottom. In the top, set the pressure to 2# BELOW the point at which the pump cuts on. So, for example, if the pump cuts on at 30#, you would set the pressure at 28#. Do this with the system off and pressure drained to zero by opening a low level faucet.