Angela, negative splits do work. good luck, go for sub 1.50! (if one of your 1/2s is the GNR, don't be too disappoint ed if you don't, it's too crowded). Once you set off too fast you're in energy debit for the rest of the race.
Re use of hrm, I used it for all my training and racing and so got an idea of what my training ranges should be.
For me, a 1/2 m effort equates to about 85-86% working heart rate. So I made sure my tempo runs were run at 80-85% (at the lower end of usually).
I set my 1.45 target on the basis that my previous pb was 1.51, and it sounded like a good number!
As my training went on, the pace I was running the tempo runs got faster, and I did one time trial of 10 miles 2 weeks before the race at my target pace on a flat route where I'd measured the mile markers previously.
So by then, I knew running at 85% would get me a min/mile pace of about 7.45, which is exactly how it worked out in my race, got 1.40 (watch time). Unfortunately it's been downhill ever since!