I know, that there are allot of games like mine and I'm not going to make the best one

I'll work with platforms, so they will appear depending on the cameras speed.
I just faced problem with making that, I create them with the use of 'total_frames % 150'.
I try to use var for '150' and change it, so they will appear faster, but I fail...
One more thing, I try to create platforms randomly from the center of screen, like this:
random_seed(0);
ent_create("platform.mdl",vector(500,400 - randomly(800),-400),platform);
but I fail too...
Any ideas?