

Yes, Prism can interpolate X values from Y values entered into several data sets. Note the following: •Prism has to figure out which values are unknowns. It uses this rule: X(or Y) values are considered as unpaired (and are used for calculation of unknowns) when they are lower in the table than all XY pairs in the data table. It is not enough that the unpaired Y values (from which you want Prism to interpolate X) is lower on the table than the paired XY values for that particular data set. They must be on lower rows in the table than all the paired XY values of all data sets in the table. •Prism offers an option in the Output tab of nonlinear regression that let you choose where the interpolated X values should go (into the X column staggered, or onto Y columns). If you have replicates and have unknowns in more than one data set, choose the option to show the unknowns in Y columns. 
When you enter a Y value, and ask Prism to interpolate the X value, that X value is in the same units as the X values you entered. Usually this is concentration. If it is the logarithm of concentration, you can transform it back to a concentration scale. 
In some cases, there are two X values that correspond to a Y value you enter. To understand how Prism handles this situation, you need to know how it interpolates. It starts at the minimum X value and works its way to the largest X value. It stops when it finds the Y value you entered. If it doesn't find a Y value matching the one you entered, it then looks at X values lower than the minimum shown on the graph and to X values larger than the maximum. If there are two points on the curve that have the Y value you entered as an unknown, Prism will find the lower of the two corresponding X values. 
Sure. The interpolated values are tabulated on a results table. If you want to plot only these values, start from this table, click the New button, and choose New Graph of Existing data. If you want to add these interpolated values to the graph of the unknowns and its fit, go to that graph, doubleclick to go to Format Graph, go to the middle (Data on graph) tab, and choose to add the table of interpolated values. 