I hope I'm not intruding, but your post has made me curious about a couple aspects of string inverter design:
1) If Voc,STC = 49V, what is your design extreme minimum temperature and what is the temperature coefficient of Voc? For example, one random 425W module I looked had a temperature coefficient of -0.286%/C for Voc. So if your extreme minimum temperature is -15 C, that's a 40C delta from STC, meaning that Voc would rise by 11.44%, to 54.6V. Since the SMA inverters are limited to 600V, that would mean your maximum string size would be 11 panels (which would be 600.6V in this example, but I assume that's close enough.)
2) From the discussion of 4 strings vs 8 strings, I infer that you are currently considering (4) 7 kW SMA inverters, or 28 kWAC for panels rated at 68*425 = 28.9 kWDC. That would be an AC/DC ratio of 1.03, which I understand is lower than the cost-effective optimum. [Unless you are interested in maximizing total power produced for a fixed roof area, without regard to the marginal cost of your marginal watt.] So have you considered dropping two panels to have 66 panels in 6 strings of 11, and just using (3) 7.7 kW SMA inverters? That assumes your computation as in (1) above allows 11 panel strings. That would give you an AC/DC ratio of 1.21.
Or if you want to stick with 68 panels, you could use 4 strings of 8 and 4 strings of 9, for string DC powers of 3.4 and 3.8 kW respectively. Then you could put one of each on a 6 kW SMA inverter, for a DC/AC ratio of 1.2 Or use two different inverter sizes, either 5 kW or 6 kW for 2 strings of 8, and then either 6 kW or 7 kW for 2 strings of 9.
Cheers, Wayne