I like the one by Geocities
http://www.geocities.com/d_halem/wheel/wheel.html Its very accurate. For the front wheel yes I get 296.3. However for the rear wheel there should be 2 different measurements from the center to the flange, one from the center to the left (non drive side) and the center to the right (drive side). The right side is usually narrower to allow space for the cassette or in your case the cog. To get the right measurement measure the length of the hub (area that touches the inside of the fork). Divide by 2 and mart that spot on the hub. THEN measure center to flange, from that spot (which should be the center) this way you have the left side probably your 28 left and the right side probably 27 or 26. Your drive side spokes will be roughly 1mm shorter or so. I would guess you would need 48x296mm and 16x295mm But you did well those spokes should work.
Oh and the geocities has pre set hub measurements and rim ERDs you can select with the drop down but I usually just do my own measuring.