I've been using a spreadsheet since the start. I'd be lost without it and its twenty pages. I've got pages for the roster, deployments, future deployments, building times, my construction estimates, and other pages where I just compare different designs. The heavy cruiser page, for example, has about thirty different designs together so I can evaluate what suits my fancy.
I find - probably as Roger does - that the spreadsheet, if I project the construction program ~3 to 4 years in to the future, lets me optimize my factory assignments and optimize my infrastructure usage. Even going balls-to-the-wall, I rarely, if ever, need additional slipways to build what must be built.
The challenge right now is to get my drydocking facilities up to snuff, but that's going to be a few years in the making.