Sums...not!

So here's a thing. I'm beavering away this last weekend in the 'shop, preparing timber for Steve Allford's course and making a rather large frame for a map of the world, again for the same person at work as the other few finished recently. Sunday afternoon arrived and I had a couple of hours to spare so I thought I'd make a start on this small project to hold some mugs. It'll also have a curved front drawer, which I'm rather looking forward to, as I've never done one of those before. All the timber had been put through the p/t, the edges and ends had been shot in and the surfaces skimmed with the LV LA jack...perfect, everything looking hunky-doodly. I selected the two uprights and proceeded to mark out the dovetails top and bottom, then I moved on to sorting out where the two central horizontal dividers would go...so far so good. Marked out one (the top of the drawer) then measured 120mm for the next shelf...but hang on just a moment, this was only leaving me about 60mm for the next mug and I needed 120mm! Time to check the drawing.
The dimensions for the individual sections were as follows: 15 (thickness of wood) +75 (drawer height) + 15 (first horizontal shelf) + 120 (space for mug) + 15 (thickness of shelf) + 120 (space for mug) + 15 (thickness of wood) You don't need to be a maths graduate at Edinburgh to add up these numbers, which if you've hit all the right buttons, ought to come to 375mm...so why the bloody hell did I mark on the drawing that the total height was 236mm, which is what I cut both pieces to?
Bugger and thrice bugger!!
Staying remarkably calm and resisting the temptation to hurl everything in the 'shop through the window I reviewed my options. Fortunately, Waka who kindly donated the timber in the first place, is coming up to Wilton in a couple of weeks (along with some other notorieties from UKW) so he's going to bring me another couple of lumps of timber...here's hoping I don't do the same bloody stupid thing again. As is often said in woody circles of note, measure twice, cut once...'cept I didn't!