Since the lake is average depth of 30', i would set the range to 90 or 100'. If you are set to 60, half of your data is really just the water column. Now, if most of the lake is average 10-15' and only a small portion is 30'. That may be ok. I know this will be harder to see objects on the unit itself while scanning, but it you will be able to zoom in much better on playback and in the 3rd party viewers. I would also do my initial run at 455khz and then go back with 800khz on objects of interest. I wouldnt worryabout scanning at 1mph. At 3mph, the unit pings enough times to get a fantastic image of most anything you need to see. I would just go back with 800khz!
I use a third party software to create my runs and then convert the waypoints to gpx to import, and then create a route with Humminbird. You can do it manually as well.
I know there are some numbers here on the forum about how much data will be recorded per hour, but you may have to search to find it.
What i would do is make each run a seperate recording. And of course HB automatically stores them sequentially by name.
Then i would use one of the 3rd party programs to view or create a mosaic.
Robert