fieldlanding,
i believe most of us tend to think of each TOW equates a single set of V speeds as most of us 'calculates' our TO perf from a Specific Takeoff Chart (at least we do i.e. a tabulated chart with perf figures corresponding to each TOW at a certain ambient conditions) which is to save us time. I doubt many of us would calculate a takeoff perf using Generalised Method which would probably result in not many flights departing on time
if we really look at it carefully, for a given set of ambient conditions, there is a MTOW. This MTOW will correspond to a set of MAX V speeds (i call it MAX for lack of a better term). many a times, we are taking off much less than the MTOW, therefore we can actually use ANY speeds right up to the MAX V speeds.
Let not consider WET conditions yet. In you example, assuming 60T and 159kts is the MTOW for this particular runway under this conditions. therefore, if you are taking off at 40T, you can use a min. V1 of 126 right up to a max V1 of 159kts. just like you say, there is no reason why the acft cannot stop at 159kts at 40T when it can stop at 159kts at 60T, right?
Now, if we move on to WET speeds calculations. for FAA certified acft i.e. Boeing, there is actually NO requirement for WET V speeds calculations. it is only in recent times where Boeing included "Advisory Informations" which gives Slippery Runway performance figures. But for JAA certified acft i.e. Airbus, they do infact have Wet V speeds.
In this discussion I will stick to Boeing (as I am not Airbus qualified). To calculate Slippery Runway degradation, the correct method is to enter the table at the MTOW and MAX V1 and apply the corrections. If your ACTUAL V Speed is LESS than MAX WET V1, then there is NO requirement to apply corrections to your ACTUAL V1.
Again from your example, since the MAX WET V1 is 154kts and your ACTUAL V1 is only 126kts, you should not apply the -12kts to the V1. The -12kts correction only applies IF your MAX V1 is 126kts (which happens if you are taking off from a very short runway).
I hope my explanation makes this very ambiguious topic clearer for you and not the other way around
Happy Landings...
SR