Where do I modify sheet resistance values for layout when running Assura LVS ?

The Assura extract rules (often called extract.rul but that depends on the PDK) will identify the resistor devices and extract the device using (probably) extractRES (could be extractDevice) - you can check for the component name as the argument to this statement. Then there will be a bunch of measureParameter and calculateParameter statements after this which would compute the extracted parameters from layout. Embedded in that would be the resistance equation (assuming resistance is extracted and not just physical dimensions) and that would almost certainly have a term which is the sheet resistance. 

For example, this is from gpdk090:

r=calculateParameter(((length - (0.45 * bends * width))/ width) * 400.0)

The 400.0 would be the sheet resistance.

That said, this is a bit of an odd thing to want to change, since it's something you have no control over in reality...

Andrew.

Thank you.   I have a note from the foundry regarding the resistors I am implementing in my mixed-signal design.  Some resistors have metal buses running over them and others are completely covered .  It appears there will be an effect on the final resistance.   In some instances I am compensating for this.   I'm not quite sure the best approach.  I'd ultimately like to get a clean LVS but I need to treat some resistors differently.

This isn't something we can safely discuss in a public forum without the danger of breaking the foundry's NDA.

It would be better to talk to the foundry, or customer support at least rather than doing it in a public forum. Very hard to give general answers (other than what I've done so far) to a specific issue.

Andrew.