The recommended amount should be listed in your tv owners manual.
With that said, most will suggest returning the lost front axle weight (with tt hooked up, but no wd bars attached) back to 100%, so in your case the fa would weigh 3500lbs with the tt and the wd bars latched (hooked) up. You are ousted you are currently over the tv only fa weight, but only by 60lbs.... There are a few that will run a little heavier (like you are slcurrently set up), but that is a decision you need to make compared to the owners manual recommendation... The "old school" way was to get equal drop for both the tv front and rear axles (or slightly more drop for ra), but that changed a little over 10yrs ago or so (could be longer now...) when th tv manufactures started "raking" the trucks.
There are some trucks that it is now recommended to only return 50%, or even 25% of the lower weight to the fa, letting the rear doe most of the work.
Depending on the year of your truck the gvwr should be listed on a sticker somewhere in the drivers door jamb. Probably the same sticker as the axle ratings are on...