You can do either. Changing the outdoor temperature to the 99% outside design temperature is a relevant point on the outdoor temperature scale (+5F, in your case), then adjusting the temperature to where it's keeping up at whatever your mid-winter overnight temps happen to be (usually 10-20F warmer than your 99% number, but sometimes cooler.) When the outdoor temperature drops below the 99% outside design temperature the boiler will still continue to increase the boiler temperature if it can- it's not setting temperature limit, just a point on the curve that you are defining.
Since the response of fin-tube isn't very linear with temperature as average water temperatures drop below 115F or so, it's better to pick a minimum boiler output temperature of ~120F or so, then keep raising the outdoor temperature until it doesn't keep up.
At the warmer-outdoors/cooler water temp end of the curve you also have the issue that at an AWT of 120F a single 40' zone can't emit the minimum fire output of the boiler, and it will be cycling on/off. That cycling would only get dramatically worse if the minimum output temperature dropped to 110F or lower, taking a toll on efficiency and putting wear & tear on the boiler, so you really need to hold the line at about 120F out of the boiler to get both a predictable curve that covers the heat load at all outdoor temperatures, but doesn't short-cycle the boiler on zone calls.
When you add a zone to the basement (after insulating it first), even though it won't need even the 25' of baseboard you were thinking, increasing it to 40' would be nicer to the boiler, since a 25' section would induce short-cycling when only that zone was calling for heat.
Since the response of fin-tube isn't very linear with temperature as average water temperatures drop below 115F or so, it's better to pick a minimum boiler output temperature of ~120F or so, then keep raising the outdoor temperature until it doesn't keep up.
At the warmer-outdoors/cooler water temp end of the curve you also have the issue that at an AWT of 120F a single 40' zone can't emit the minimum fire output of the boiler, and it will be cycling on/off. That cycling would only get dramatically worse if the minimum output temperature dropped to 110F or lower, taking a toll on efficiency and putting wear & tear on the boiler, so you really need to hold the line at about 120F out of the boiler to get both a predictable curve that covers the heat load at all outdoor temperatures, but doesn't short-cycle the boiler on zone calls.
When you add a zone to the basement (after insulating it first), even though it won't need even the 25' of baseboard you were thinking, increasing it to 40' would be nicer to the boiler, since a 25' section would induce short-cycling when only that zone was calling for heat.