The 202 is a very forgiving powder, even though the 155 Scenar is a lot longer than the 155 Sierra that is listed with Norma I would go with about 42,0-42,5 grains 202, standard primer and a COL that is longer than in the loading data here:
308 Winchester - Norma
My experience with the 203-b is that it is a lot less forgiving, pressure will spike hard when the loads get to hot, however the load data above states that a 168 SMK can be loaded with 44,8 grains of 203-B and that with a WMLRP, so in my perspective I would not hesitate to start with the 155 Scenar at around 43-44 grains, standard primer and a COL that is longer than suggested.
Note the last time I shot the 155 Sierra Palma and Scenar I just substituted the two and used the load I had from the Palma to the Scenar at the same COL the Scenar load was to hot however given the same lenght to ogive and the same seating depth as the Palma it worked out fine, even though it was a single feed ordeal to shoot the Scenar. That bit speaks volymes about the freebore I had in my old sako 308 Win barrel.
Out of the two powders the 203-b will give better velocities still some people here just can not seem to make it shoot well enough, the 202 is designed for the 308 Win case and a load of 42,5 grains is a lot like that 43 ish grain load of Varget that I usually read about here, producing great accuracy with a great number of bullets.
Well that is my 2c worth /Chris