I hate to jump in on a older thread, but this question (and an answer or two) seem to indicate a shallow understanding of what a WIP Limit is and is used for.
The WIP Limit policy is poorly named if you are doing anything more than a shallow kanban system implementation. We used the term WIP Limit (the actual name) originally because we were so overburdened that we needed language (specific words) about a policy that reduced overburdening. In that situation WIP Limit may be considered an appropriate name/label for the policy.
Deeper kanban implementations stop thinking of reducing overburdening (which is a significant and positive side effect) and prefer to think of economically optimizing flow. The term Optimal WIP starts becoming a more appropriate name for the policy.
Most people think that WIP policies are only intended to stop push, which they do, but they are also a signal to the producer to pull. If you are cronically under your WIP policy number, you should lower it.
As to the original question, the optimal WIP matches the arrival rate of work into a queue with the departure rate. You may need to step back and look at the overall system and not individual phases in your workflow to see this appropriately.
You may also be experiencing emotional resistance to a policy that would be beneficial if properly observed. I'd start with asking the team members why they aren't able to follow a policy that is intended to promote good behaviour.
I'd also add that WIP policies are not laws. They are guidance to what a team thinks is good behaviour. Exceeding them is fine as long as the decision to make an exception is informed and actually an exception and not the norm.
Check out my blog post for a more complete introduction to this idea.
What Should My WIP Limit Be