Since SMRT does a simple straight-line interpolation between the two nearest points, the solution is to figure out what % between $25 and $50 is your new value, $35. It is $10 on the way along the $25-point range between $25 and $50, which is 40% of the way.

So, let's say the utility reported in SMRT for level $25 is 80 and the utility of $50 is 40. To compute the interpolated utility for $35, you multiply (1-0.4)(80) + (1-0.6)(40), resulting in 64 utiles.

To double-check yourself, do a scatter plot of the three prices with their three utilities in Excel, and it should be a straight line.

Some people build their own simulators in Excel and the formula I just gave is the easy way to do the interpolation for computing the utilities at the individual levels for the raw utilities prior to summing the utilities across attributes (to obtain the total utility of a product concept) and exponentiating the utilities to obtain the Share of Preference per the Logit rule.