For a no-sparge, you can get a really accurate estimate of the pre-boil gravity as long as you assume 100% conversion efficiency. Given that, the only factor that impacts SG is the liquor-to-grist ratio. Kai has a chart here: http://braukaiser.com/wiki/index.php?title=Troubleshooting_Brewhouse_Efficiency#Determining_Conversion_Efficiency
Since you want to dilute to 5 gal at 1.050, you need to collect 3 gal at 1.085. Checking the chart, you can see that requires a mash ratio of 1.44 qt/lb. Assuming 0.12 gal/lb absorption and no dead space, x is the grain weight in pounds:
3 + 0.12x = (1.44/4)x; x = 12.5 lb
This is really helpful. It is also giving me an idea of how I can do my Wee Heavy this year. Since I don't drink a lot of it, a 3 gallon batch would be fine. It won't be so expensive and I can use a 5 gallon bucket for a fermenter.