Just divide the lock settings by same number the steering range setting was divided by,
Eg. If 900 degrees rotation was used with 30 lock, then 450 degrees would be 15 lock - they're both divided by 2. If 1080 rotation was used with 33 lock, and you want to know what lock to use for 180 degrees well just find out how you got from 1080 rotation to 180 rotation. To get from 1080 rotation to 180 rotation you divide by 6. So you do the same with 33 lock - divide it by 6 - and you get 5.5 lock.
The thing is, with only a tiny amount rotation (180), it will be very difficult to use a realistic steering ratio because you have such a tiny amount of rotation. So what you will almost certainly need to do is use a higher amount of relative steering lock; this will make your overall steering more touchier/faster. Then, to try to compensate for your faster steering ratio, you can lower the steering sensitivity in the game's controller options; doing so will slow down the first half, or so, of your steering ratio so that the first half or so of your wheel-turning will behave as if you're using a slower steering ratio.
Why are you only using 180 degrees of rotation? Are you only counting one way? Even the Logitech Momo Racing - a budget wheel from over 10 years ago - has more than 180 degrees of rotation (240 degrees).