Renovation Cushion

[Shaquetta Chittams]

What percentage of my rehab budget should be set aside as a cushion. Right now the renovation budget is at 45k and we haven't opened any walls, looked at the roof or the furnace. We know we want to update the kitchen, two bathrooms and the basement. Are there other items that should be priority over anything I listed. Does anyone have their own special formula they follow for each rehab?