I am by no means an expert on this subject but have a boeing document that has some basic figures that may be helpful to this discussion.
I'l cherry-pick some statements;
"The manner in which the additional 2 seconds of time in the RTO trasition segment has been accounted for has changed over time. Prior to 1981 one second was added to the second and third pilot actions (throttle cut and spoiler extensionrespectively), during which time the aircraft decelerated."
There was an ammendment (25-42) where the distance became 2 seconds at continued acceleration. Then there was another ammendment (25-92) where the 2 seconds is calculated at V1 speed, to my knowledge this is the current state of affairs (I'm happy to be corrected).
"The distance associated with this 2 seconds time for the 737-800, 777-200, and 747-400 is approximately 500-600 feet"
The requirement to factor the wind by 50% or 150% (head or tail) also provides a 'factoring element'.
A 737-800 at MAUW and 30 degrees celcius using idle reverse 2 engines should give you 160ft extra, detent reverse 290ft extra, and max reverse 300ft extra runway. That is of course for a dry runway and I realise the question was about a wet runway.
Like I said, I am no expert, these are things I have gleaned from one document and I look forward t the input from the likes of old smokey.
Framer