I was planning to install 3/4" by 5" hardwood planks on top of 5/8" ply over an unfinished basement over my entire first floor. After inspecting the 40 year old subfloor and thinking about how much I've spent on the wood already, I've decided to add an additional 1/2" of plywood to the subfloor to help stiffen and even out some of the dips (adding shims/felt to level a few spots before adding plywood).

I know that I should stagger the seams and glue/screw, but how much glue should I use? Run the adhesive around the perimeter of the plywood board with a few dabs in the field? More? Less?

Also, how long should the screws that I use be? I figured with the existing 5/8" and new 1/2" ply, the minimum should be about 1 1/4" decking screws. Should I go longer (maybe 2") and try and hit the joists?

Large swiggles of glue all the way across the plywood. Screws sound good, I wouldn't try and hit the joists through the second layer.