IMO even if you/they opt to enter D:P in terms of Mandays as described (as opposed to Hours) you would still be best served maintaining the Total Hours columns, you can calculate this figure off the manday values using:

Q3:
=SUMPRODUCT(INT(D3:P3)*8+MOD(D3:P3,1)*10)
at which point R3 remains as outlined previously:

R3:
=INT(Q3/8)+MOD(Q3,8)/10
If you remove the Total Hours columns and opt against Analysis ToolPak dependency (wise) then you're left with a certain amount of double evaluation.