Any Grip diagram is only an indication of approximation, that is to say diagrams are "about right". Inside any of the named grips there can be several slight (millimeter sized) variations giving a different "feel". If you use someone else's racket (same model same size) it will feel different, that's because the owner has worn a saddle or hand imprint into the urethane that supports the frame underneath the grip.  

TIP. Don't let other people borrow your playing rackets as they may change the "feel" of your grip by wearing their own hand impressions onto your grip.

The modern method of explaining grip location is by locating the pad on your hand (see diagram 1) and then locating it to correspond with specific grip locations. Specific grip locations will work better and have certain advantages with different shots. Your local Pro will be able to guide you (better than I can show you) to the right grip for particular shots.

Grips have strange and confusing names, but finding the right grip is what's important not the name of it, so you have to learn them by site and most importantly by feel. Most Pro's could hit any shot with any grip and most have a range of grips to choose from for any given shot. i.e. a forehand volley can be hit with anything from a eastern backhand grip to a western forehand grip, it just depends on the situation. The best forehand volley grip is most likely an eastern forehand grip but it may not be available in a quick volley exchange, so you may have to compromise in order to get the result your looking for.

Grip pressure is an area not often taught. It's very difficult to quantify grip pressure but the majority of strength will come from the thumb, middle and ring fingers. Your index finger is a control finger and your pinky is pretty much along for the ride. You can test the load on each finger by trying to play a shot leaving a different finger off each time. Grip pressure should be about firmness and not "white knuckleness". Another idea is to grip the racket about as hard as you would a jug full of water, say two kilos of weight (for an average man).

The Palm pad location marked with a yellow arrow corresponds to the diagrams below also marked with Yellow arrows.

On Double Handed Backhands, the Left hand should be placed in a comfortable position beside (touching) the dominant hand. The hands work together and are not meant to leaver against one another. Your coach will fine tune this to minimalise left hand wrist snap through the shot.

Note: The focus regarding double handed grip alignment is on being comfortable in order to developing feel, control and economy of shot. Players will develop specific Grips for specific shots with experience and those grips will most likely be unique to that player.