When the full assessment is done, you'll get a hefty bill called a "Supplementary Tax Bill" which will be all the makeup charges dated back to when you took possession.
Since the Milton tax rate is just over 0.8%, and you paid nearly 700k for your home, AND the assessed value of new is usually about 15% behind the actual value, your safest best is to assume:
1) Your annual taxes are ACTUALLY closer to $4400-4700/year
2) You WILL have to pay this amount at some point
3) Instead of approx $100/month, you should be allocating about $400/month in your budget for this based on the numbers you provided.
Please remember, you WILL have to pay this anyway. When the Supplementary bill comes, it will be one BIG chunk , they will give you a couple of installments you can pay it in, but you can't avoid it
If you want to PM me the name of the model you bought, or the actual purchase price, I can give you closer estimates