Q Loss (BTUs) = Surface area x (indoor - outdoor design temp) / Effective R-value*
*You have to take thermal bridging through studs into consideration - an R12 wall has a real thermal resistance value of less than R12.
The R value of basic wood double pane windows is around 1.7 if I remember correctly.
For air leakage:
Q sensible (Btus) = 1.09 x airflow CFMs x (indoor - outdoor design temp)
A blower door test is required to measure leakage.
I don't know the formula for latent heat loss off the top of my head.
Don't worry about passive solar heat gain for heating since the system has to be sized for design conditions. (No sunlight)
For making a template I recommend excel.
I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.
Death is the messiah. Everything else is irrelevant and arbitrary.