Dimension round slot from ends, not centers.

Edit: I realized I was asking the wrong question and the drawing snippet I attached doesn't actually show the dimension I'm looking for. Read on for updated info.

I need to find the distance from an end of the round slot to the origin. I've got to imagine I'm missing something pretty obvious...

I've got a part with 18 round slot "pockets" (.270" wide, .150" dp, various lengths) circling the OD of a cylinder. Their positions in Z are dimensioned from one end of the slots to the origin. Can I create planes tangent to the end of each slot and perpendicular to Z?

While I'm here, how do I dimension the depth of the slot from the highest point on the OD?

Don't cringe, been using 2011 Mr1 and now 2012 Mr1 lightly for about two years, still very much a newb. I'm comfortable in Summary mode, starting to claw my way through Command mode. Had a couple days on-site training from Colin and besides that, been figuring it out as I go(I've asked the bosses for more training). That being said, I'm stuck on these slot locations. Any help is much appreciated.

Thanks