Thanks for answering,
We manufacture, and as a more or less real example on the numbers product A uses 30 of B to be produced (alongside many other products), but product C uses 20 of B but is always produced in bulk of 20, so we don't want a stack of 800 to cover for product A and whenever a sale for product C comes, we just want to have 300 in stock (for A) and trigger a buy order of 400 of B whenever a sale for product C is confirmed.
That happens for many products, it was just an example.
I get what you are saying but that would not work for us...

Not sure why you would need inventory for 800.

Based on your example, if you were to set the replenishment to 60/300/30 to plan stock to produce between 2 and 10 product A.

While you receive order for product C, there would never be 400 in stock, so it would reserve available stock and create replenishment for the reminder. This would also mean that the stock in hand would be below minimum so it would also create a replacement to top-up stock to 300.

In effect you would have achieved MTS and MTO

I'm sorry if I'm not clear enough, thanks for your help, I've oppened a new question trying to understand why the very basic is not working... If you want to take a look at it I'll be gratefull, and if not, thanks for help so far anyway!!

https://www.odoo.com/forum/help-1/why-po-and-mo-are-not-triggering-lead-time-problem-209796