it allows 20 more moa to your scope adjustment, it wil allow you to get out to 400+yds with subsonics, otherwise you will never hold your zero out that far.

are there any drop charts for these subsonic loads?

Any suggestions on what would be a good grain weight bullet to start with?
same with powder, some seem to be a little more forgiving than others

I started with Alliant 2400, but I like Vihtavuori N110 now. It seems to be more consistent for me, but I am also shooting out of a 10.5" autoloader. N110 is extruded and seems more resistant to external conditions to maintain consistent velocity.

Using a 220 Grain SMK, Hoser's Lake City Brass, and CCI 400 Primer I use the following for subs:

8.1 Grains of VV N110

8.5 Grains of 2400

These may run fast or slow in yours so use them as a starting point and check to make sure they exit the barrel.
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Vaporizers

I have had good luck with lc brass federal gmm primers hodgon110 and 220/240 smk's with my bolt gun. Been wanting to try some 208 amax but cant find them.

Also egw makes a decent 20moa mount for around 30 bux or so

It will allow 20 >more< MOA vertical on your scope. But how far that will take you depends on the particular scope. The base just offsets whatever the range adjustment a given scope already has. Some scopes may only have 30 moa or less total vertical adjustment raange. With a scope like that a 20 MOA base might be too much to let you zero at short range. A Leupold 16x40 Mk 4 is an extreme example of a scope with 140 moa of vertical adjustment. it could use a base with 60 moa of wedge and allow you to shoot with a full two degrees of barrel inclination over the line of sight. That's about what's needed for high BC medium velocity rounds, like trying to shoot a 50 BMG using VLD bullets to 2500+ yards, or a subsonic at 1000 yards.

Some scopes have reticles designed so the "zero" aiming point is offset vertically. The Horus Falcon with it's H37 reticle is designed to use a 20 MOA wedged base just to zero it at short range.

Before you rush out and buy a wedge base do the math to find out what you want and need. Start by determining the trajectory of the bullet/cartridge/rifle/atmosphere you'll be using. Then decide what the maximum useful range that rifle will yield. That range may be limited by velocity dispersion (typical for subsonics), by wind deflection (typical for low BC bullets), by bullet stability (typical for supersonics) , or just because you don't want to miss (typical for hunting). Then pick a scope which can cover the range of elevation angle you'll need, and only then pick a wedged base if it's needed. Leave some room at both ends of the scope's elevation adjustment range. Most ballistics programs will do the math but you have to keep in mind what a wedged base does for the situation. Drawing a sketch may help.