1. The first thing you need to do is open your console (in-game) using the: ` button.
2. In this console you should type: com_maxfps 125 you do this since the game puts too much energy into maintaining a high fps (frames per second)
so by limiting it is gives you a little boost in your jump.
Some also like to jump by sprinting using shift+W (W is key to go forward) and then when they are about to jump they hold the A key(key to go left) which apparently also gives your jump a boost.
in some situations when you need a rpg to get up onto a ledge after altering the console you can simply look up jump and press crouch(in mid-air) and you land on the ledge.