create slider for a symmetric confidence interval

Hello! I am trying to figure out a way to elicit respondents' beliefs about both a mean and a variance. So for example, I want to ask them to report both their guess about a mean of the distribution AND a symmetric confidence interval around that mean (i.e. I believe the mean is 30 and I am certain the true value is within +/- 5, so in the range of 25 to 35.) But I want them to be able to report these values visually with a slider. Is there a way to create a slider that essentially displays with 3 points (the respondent chooses a min value, average, and max value) but where there are 2 choices to make, one of which is the centering point (average) and one of which is the symmetric window around that value (the value that determines the size of the symmetric window around the average and therefore the min value and max value). I want to make sure that I can extract both of these choices.
The other wrinkle is that I want to constrain the max value to be under a certain number (i.e. no higher than 100). So depending on the average value they pick, the symmetric window will only be able to be so large.
Any thoughts about how to best accomplish this would be greatly appreciated!