help with perspective

[darktiste]

thank you for the great answers,i have already studied successful drawing and i find myself struggling with perspective grids,it's the part that i don't understand,expecially the reasoning behind the measuring point,loomis seems to just place them randomly when it comes to creating perspective grids

You can find more about that in scott robertson book 
Drawing and Sketching from imagination with scott robertson and if you can't find that you can probably find people who would explain it on youtube you just need to type something like perspective grid technique and theory something similar.