Bob's calculation is correct.
However the question to refinance should be based on your answer to this question? "How long are you planning to stay in the home? If you have long term plans or if you have no plans to sell, you should consider refinancing. Anyway the decision is yours. Rates and home values go up and down keep that in mind!!!
If not this would be a good time to refinance into one loan with PMI or LPMI.
You may be able to get an appromixate idea of the value of your property at:
If the 1st loan is an ARM, then you need to see when the interest rate changes and by how much to determine if you should refi now. If the rate changes soon, then doing a refi now could be of benefit while rates are low.
If you would like more information I would be glad to discuss it with you. I can be reached at 559-325-3650.