Please help LOL my head is exploading with the numbers!  I am not good with numbers.
		
		
	 
I love Excel.  When I was wading into how much to buy, and how much my pocketbook was going to take, it was great.
Laying "how much" out by hand isn't too bad.
Weeks 1-2-3-4 = 4 weeks at 2.5mg = 10mg
Weeks 5-6-7-8 = 4 weeks at 5.0mg = 20mg
Weeks 9-10-11-12 = 4 weeks at 7.5 = 30mg
Weeks 13-14-15-16 = 4 weeks at 10mg = 40mg
Weeks 17-18-19-20 = 4 weeks at 12.5mg = 50mg
Weeks 21-22-23-24 = 4 weeks at top dose 15 = 60mg.
Maybe you're seeing the total/month (4 weeks) has a pattern.  Not much to do with anything but is cool to see. 🙃
SO... For the first 24 weeks you'll take 210mg of Tirz *IF you titrate up on the research study's schedule.  (Many do not but that's another discussion.)
Which tells us some things.
1. If you're looking toward a kit of 20mg vials : 20mg x 10 vials = 200mg which gets you almost all the way through 24 weeks.  We know that the first vial is going to last 6 or more weeks.
2. Some people prefer to use up a vial in a month. 20mg vials would last longer than a month only* the first month.  If you buy a 60mg vial, it would take 3 of the early months to work through - that's 12 punctures.  *IF you're on research schedule.  Understanding your risk and what you want/don't want is important. 
3. Another case: if you get a kit of 10s, you know 10x10mg = 100mg will last about 16 weeks.  If you need to order more you'll need to know timing IF you end up not hoarding vials like a few of us. 🙃  
4. Many 10s would mean upper doses would take multiple vials per dose.  Two vials per needle dulls the needle - again looking at risk and what you want/don't want.  (There are gear (hardware) workarounds, bringing more $, more education.)
All of the above is how I decided vial size for me: 10s and 20s seemed good to start with, mayyybe I'd consider 30s if it was an amazing deal.  It was also about the price point I was willing to put out for a single purchase.
This is also how I compared 16 weeks of Reta vs Tirz vs .... ect. which is what started it all.  Which is least expensive? How do I compare when it's 2.5mg to start this one and 0.5mg of that?