Counting half or 50% of a fluctuating range of cells

So, I'm trying to make a sheet that will calculate handicaps.

I need no more than 20 cells across per person. But I need the formula to calculate the average of the lowest 50%.

10 numbers in a range, I need the formula to grab the lowest 5 values then divide them by the cell count.
Example: Grids.png


But as numbers are added in the row that may be lower I need it to keep calculating.

Example: 12 numbers will then take the lowest 6 values and find the average.
17 numbers, will take the lowest 8 values.

I think it's going to required 2 pages as a reference. In my mind I have a Find lowest n value in a range, but I need an IF formula to somehow choose how many cells to add together.
Like if there are 10 cells, add cells 1 to 5... I don't even know it that's possible..

any help would be great!