How to Repair a Storm Door

Hi, I'm a little puzzled by the final steps in an instruction pamphlet for installation of Jeld-Wen aluminum windows. I'm trying to figure out why I would want a 3/8" gap (see below, copied from the instructions) between the window and the siding, and how the exterior trim is installed to seal and hide this gap. I haven't installed windows before. The rest of the house has 2x2 cedar trim around the windows. I'm a little uncomfortable with hoping it'll all make sense as I do the installation and would like to understand how this works before starting. The windows have a slip-on nailing fin on the inside and another lip or fin on the exterior side of a j-channel. Should the trim cover this exterior fin? The full instruction sheet is at [url=http://www.jeld-wen.com/resources/installation.cfm]JELD-WEN Windows and Doors | Installation and Finishing[/url] (New construction aluminum windows (pdf)). Thanks! Steve From Jeld-Wen website at [url=http://www.jeld-wen.com/resources/installation.cfm]JELD-WEN Windows and Doors | Installation and Finishing[/url] (New construction aluminum windows (pdf)). AFTER INSTALLATION 4. Install exterior wall surface within seven days of window installation. 5. Maintain gap of 1/4"-3/8" between window frame and fi nal exterior wall surface (siding, stucco, etc.). 6. Seal this gap on the sides and under the window sill with backer rod and sealant. Do not apply sealant on top of the drip cap 7. On the interior, insulate the void between the rough opening and the window frame with loose-fill fiberglass insulation. Do not use high-expansion foam.

We have a turn-of-the-century custom Mission style wood door that is 2 1/2 inches thick and with the half round top. It fits reasonabley tightly in the entrance way except at the bottom where it is pretty rough. The problem is air movement at the bottom in winter. Who nowadays can do this type of door repair? Are there special finish carpenters that can do this type of rebuilding? I had even priced out over the Internet storm doors with a fine wood frame, half round top, almost all glass that would properly display the main door. These start at $1000 and go up. I had also looked at these spring mounted door sweeps that will pop up when the door is opened and then pop back down when it is closed to maintain a reasonable seal at the bottom. What do you suggest?