Adapting 1st Edition Battlesystem to Ascending AC

I always been a fan of 1st edition Battlesystem. Namely because out of all the miniature wargame ruleset for DnD that have been produced I feel it is the one that operates the most seamlessly with ADnD 1st edition. As well as intregrating individual heroes well. There is nothing like seeing the true power of a fireball when you chuck it on a incoming formation of orcs. With makes even more nice that there are notes that tell you how to make it work for plain DnD namely the Mentzer's BECMI line.

I am probably going to use it for an upcoming game using Swords & Wizardry and my Majestic Wilderlands. Now the original Battlesystem used THACO and descending AC to calculate how much damage is done each round. So how do I use it with a retro-clone that has Ascending AC and To-Hit bonus?

Normally it works like this.

2d6+ THACO-AC + modifiers.

I played around with the numbers and found this works

2d6+Ascending AC-To Hit Bonus+modifiers.

So if you have a THACO of 17 and trying to AC 8 the original formula will give you +9
17-8 = 9

With Swords and Wizardry, d20 or any other retro clone using Ascending AC it would work like this.

A THACO of 17 means the character has a +3 to hit bonus as AC 0 in Ascending AC is 20.
AC 8 means AC 12 in Ascending AC so the formula is now

12-3=9

The rest of the Battlesystem rules with dice of damage, modifiers, even the unit stats work pretty much as is with anything from ODnD to 3.5.