How to calculate split for 50% and 75% power

Question
QUESTION: Hello, our rowing coach asked us to do several pieces this morning 6'@ 20spm, 2' @ 50% power, 2' @ 75% power and 2' @ 100% power. Since pressure on the erg stretchers is really not finite but rather a feel. How do you know you are doing 50% or 70% or 60% for that matter. Is there a calculation for split time that would indicate that I am at 50% or 75%?

ANSWER: Hi Paula: Interesting question. On the water, there is no clear answer on what half-pressure and 3/4-pressure is.  It is a feel kind of thing that you know through experience.  I once heard from someone you can carry a fairly normal conversation or even sing at half pressure, a strained conversation at 3/4 pressure, and you should not be able to talk at all at full pressure.

I think with the erg, it is a bit easier to determine percent power.  Let's say you can do a 2:00/500 at full pressure.  Half pressure would be anywhere from 2:15 to 2:30.  Three Quarter pressure is around 2:07.5 to 2:15.  You can probably do the math to determine how that equates to you.  

Good Luck/AP

---------- FOLLOW-UP ----------

QUESTION: Thank you for your response. But how do I calculate for my split time. In other words what is the math equation and what is it based on?

Answer
Hi Again:

Let S = Your Split Time in seconds

For 50% Pressure S(P.5) = S + (S * .2)

For 75% Pressure S(P.75) = S + (S * .1)

Like I said in my previous post, this is based on mostly my subjective feel of what 1/3 and 3/4 pressure feels like to me on the erg.  Your coach may have an altogether different concept on pressure and splits.

Regards/AP