At least this you are not afraid to open a whole new can of old worms on this one! This subject has so many variables that you will never decide on a correct course of action unless you make some absolute assumptions about many things using insufficient data. And those points will never be agreed on with the group that reads these discussions. The term that comes to mind on calculating these items is SWAG. My input is to build in a lot of FLEX on the budget when you set it based on the ROI & TCO study you do because you will always get a surprise on the project.
Don't take Tony's reply personally. He is an extreme detailer and serious information sponge. But hey someone has got to be.
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