Several techniques are available to assess the reasonableness of the testable requirements estimates.  This assessment is recommended to refine the estimate and get buy-in from the other project team members.

Review with Peers
To assess the reasonableness of the estimate once it is prepared or even during the preparation process, review can highlight omissions, errors, unrealistic assumptions, etc.  Review can also help achieve agreement and support among other team members for the estimate.
Compare with Available History
If comparable history of size measurements from other projects is available, it can be very useful to assess the reasonableness of the estimate.  It may also point out areas that may have been overlooked.
Evaluate Confidence Level
The estimator's review of the estimate is also valuable.  The areas in which the estimator has less confidence in the estimates are likely to be the areas to be highlighted as risk areas.
Justify with Assumptions
Comparing the assumptions to the estimates can show where the assumptions support the estimates or where more explanation is warranted.  This assessment can also point out additional undocumented assumptions.
Compare Relative Size of HLRs
If during the review of the estimate, an HLR that was assumed to be relatively small requires a larger number of testable requirements than expected, it should be examined.  This may be a project management perception issue, or the requirements are actually under or over stated.
Evaluate Risk
Risks are highlighted by illustrating areas of the system that are poorly defined, and/or areas where the size is out of proportion with the other estimates or is unexpected.  Furthermore, testing coverage should be aligned with risk by identifying areas where the planned testing coverage does not reflect the relative risk levels of the requirements.