Hello Babynus,
By planned periode I expect that an activity in plan for a periode (start date and end date) then the resource availlability must be calculated for this periode, it could be an average of availability eg :
Activity1 W4 to W7 effort = 40h (W4 = 10, W5 = 10, W6 = 10 and W7 = 10)
Activity2 W6 to W8 effort = 40h
Resource1 already affected to Activity 1 (capacity = 0.9)
During the affectation process the Resource1 will display is availlablility at this calculated value :
Sum [activity afected to resource1 in Weeks of Activity2] 10 + 10 + 0
Capacity *
= 0.9*
= 15%
Sum [Availlable hours per week for Resource1 in weeks of activity2] 40 + 40 + 40
then the Resource1 is loaded at 15%, then the Activity2 could pass (CQFD

)
I think this calculation is not easy because you could take account the availlability for each week including holiday calandar. Instead of display the load in % you could show the number of availlable hours (85%*3*40*0.9) = 91.8h open hours for activity2.
Have fun to implement this
BR
Thierry,