So I recalculated my TDEE/fasting calories, started the 30 Day Shred, and am setting off with this new plan. I was close to giving up, but now I have a spreadsheet and everything, plus some renewed vigour.
25% of my TDEE actually puts me at 416 calories or so - I've heard mixed advice about whether we should go below 500 calories at all, so I'm going to continue at 416 and see how I go.
Wish me luck!