I am trying to run a power calculation for a randomized control trial. For this I need a mean and standard deviation for our 'baseline'. There are papers out there which would have a mean and standard deviation, but what we would want would be to use their final outcomes as our baseline. In other words, these papers applied treatment $A$. I would like to apply treatment $B$ on top of treatment $A$, so treatment $A$ outcomes will be my new baseline.
In looking at their results tables, I can find the mean and SD of their baseline, plus the coefficient of the effect and that coefficient's SD. To find the new mean of my baseline, I would add the coefficient to the mean. However, is there a way to back out the SD of the new mean from the information I have from these papers?
To give an example: one paper has the mean in control as 5847 (SD=16784). The change of the mean under treatment $A$ is 267 (SD=527). So my new baseline would be 5847 + 267 = 6114. But would there be any way of getting this SD without the data? (Almost all of it is not available publicly.)
Thanks so much!