While it's not impossible to get natural vacuum in 5/16 it would be very rare. The reason, is that in 5/16 the air (gases) can pass the sap, thus hard to generate vacuum in it from gravity.
I'd suggest either of 2 choices, If the slope continues, either extend the 3/16 all the way, or use a 3/4" mainline. The 3/16 will ensure you have max. vacuum on every tap, the 3/4 main line will carry the sap without backing up in the main and reduce your flow.
If calculating your costs, include a saddle fitting for each lateral entering the main, a hook connector and the price of the support wire along with the twist ties and your time to install it. Don't forget to add the cost of an anchor at each end, and likely posts or side ties. Personally I'd favor 3/16 all the way just for the ease of installation and the cost of those extra items.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.