A downhole assembly, such as a toe-to-heel gravel pack assembly, has a body with a body passage, outlet ports for slurry, and screens for fluid returns. An inner string deploys in the body to perform the toe-to-heel gravel packing. A telescoping adjustment device allows the inner string to space out properly when deployed to the toe of the assembly. Sealing surfaces of a locating device in the body separate a sealable space and seal against seals on the inner string movably disposed therein. Fluid pumped in the string produces a pressure buildup when the string's port communicates with the sealable space. The pressure buildup indicates that the tool is positioned at a first location in the assembly, and other positions for placement of the tool can then be calculated therefrom.