Connector Allowance for length?

If I want 3 ft between centers of a run using a number of Cross or T- connectors, how ling do I cut the pieces of pipe? What allowance do I need to consider for the amount of pipe in the connector and the distance between the connector seat and center line of the T or Cross connector?

Here is a formula I use to determine the length of each segment between connectors This equation helps calculate how many equal segments you need to cut to span the desired length:

Segment length = [Desired length - (total length of fitting x no. of fittings) + (insertion depth x no. of fitting ports)] / desired number of segments

I explain this more in the "Designing Your Project" section of the Tools and Materials lesson.