Ok, here comes the maths bit...
Aptamil tub says to use within 4 weeks. There is 900g in the tub. If you use 180ml/6 scoops of powder per day (the recommended amounts per bottle from 8 weeks+) that's approximately 770g per 4-week period. If you wanted to use the whole tub, you'd be looking at getting through it in about 4 and a half weeks. I combination fed from 6 weeks (just 60-80ml per day initially) because of weight-gain issues and never realised there was a 4-week expiry (bad mum alert!) and I don't think it had any adverse effects, however if I'd realised I probably would have discarded the tub after a month. I wouldn't worry about using it for just over 4 weeks personally.
The cartons are 200ml and have to be used within 24hrs, so using the example of 180ml above, you would get through 1 per day. By my calculations, 1 carton per day would work out at over £20 per month (cartons cost 69p in Tesco) whereas the powder is around £9 for a 900g tub. So, the tub works out considerably cheaper.
Although the powder is a bit of a faff, I found I still had to warm up the carton milk (rather than waiting for it to cool down), as my baby did better with warm milk, so it wasn't much easier. The info posted above about water temperatures is a bit confusing IMO - the general instructions are to add the formula powder to the water 30 minutes after it's boiled (as I guess it's at 70 degrees at that point).
The only general thing I would say about combination feeding is to make sure you get breastfeeding well established first and stick with exclusive bf for as long as you can. You might also want to consider expressing so you can get help with feeds in the early days. Depending how long expressing takes for you (and how long your baby feeds for), this could also speed up the feeding process (and you can express while doing other things once you get the hang of it!) You can also continue to express after you introduce formula to maintain your supply (freezing the milk if not needed straight away). I found bedtime was the best time to give a bottle so my partner could help out - also, this is generally when your supply will be at its lowest (once fully established).
Also, don't listen to anyone who says combination feeding is a slippery slope - I am still breastfeeding my 11mo and love it! I didn't think I'd ever say that in the early days - expressing and introducing a small amount of formula helped me to carry on.
Good luck and well done for being so organised! I didn't think about any of this until my baby was here!!
p.s. my DD had tongue tie - always worth asking to get it checked - both after birth and once you're home (not all midwives are trained to spot it).