Must admit I'm a bit of a sucker for a good spreadsheet too Alison! Nice to see the progress all laid out in front of you. I've been using a weight tracker app for my iPhone which is quite good. I don't suppose it would allow the forward planning though. Mind you, I've not checked. Maybe I'm underutilising it. That's a good goal you've given yourself though. I hope you achieve it.
For myself, I tend to ignore BMI. Don't believe in it at all. Although I'll be the first to admit I'm overweight I'll never be aiming for what the BMI tells me is a heathy weight. If have to lose another 6 stone and with my body shape the way it is, I just don't think it'll be possible. I'd look ill I reckon! It does work for some people but those are the people closest to this hypothetical average which some mathematician worked out way back when. For example, I used to do a lot of weight training and put on weight and bulked up during that. BMI however, doesn't take in to account the lean muscle mass on a body. This means that all your Olympic sprinters will be in the'overweight' category even though they have no fat on them at all.
Sorry, nothing more guaranteed to get me to rant than BMI. I've fallen afowl of it in the past with medical a for my work and have many a discussion with the doctors. I'll take some deep breaths. Lol.