Visit for 6 months out of 12 on a rolling 12 month basis?

If a non-visa national has spent 2 months (July & August) in the UK as a general visitor and wishes to return for a 6 month visit, how soon can they come, bearing in mind they should not spend more than 6 months out of 12 there? Could they start a 6-month visit at the beginning of March, as in the ongoing rolling 12-month period they will not be there more than 6 months? That is, at the end of August they will have been there 6 months of the previous 12 (and 8 months of the previous 14). Or, at the beginning of March would they be told they could only enter for 4 months as they had already been there for 2 months? (I know it is actually 180 days, not 6 months. I'm just rounding it off for convenience.)

There are no hard and fast rules. Only a six months in twelve policy. It very much depends on the IO/CIO.