Here's the dimensions of a "full" Ultimate field. Of course, we often reduce these to fit onto soccer fields and avoid goal-posts or cricket pitches. (100m of open space is rare) But the magic numbers are 64, 37, 18. Say that to yourself 10 times and drill it in. 64, 37, 18. 64, 37, 18. 64, 37, 18. 64, 37, 18. 64, 37, 18.
Also, check how far the brick mark is. 20m. That's a lot further than I usually see in Brisbane Ultimate. Almost 1/3 of the field. Its that size for a reason - to punish poor pulling.
Often though, after having gone and retrieve a disc that's gone out on the Pull, we're not so keen to jog over to the centre of the goal line, then take 20 steps out. Its a detour, and you look a bit lame (maybe we should consider some small or different coloured 'brick cones' for BPL).
Here's a bit of a rule of thumb. Since we usually have half-way cones for some reason, imagine some diagonal lines joining them to the front end zone cones. Put yourself where the diagonals cross (ie, where you think you're about the same distance from all four cones), and you're 1/4 of the way down the field (about 16m if you've got a full sized field). Then take another few steps and you've got your real brick mark.
Edit: For those who don't quite get it, here's a picture. A further tip for making it look like you know what you're doing is as follows: after you've retrieved the disc, bring it to the front endzone cone, sight the half way cone and walk towards it, until you've crossed the other imaginary diagonal (or feel you're halfway across the field). Then take your four steps forward.