Bornfree, the theoretical value of time in the options is as calculated by you. It does not deteriorate in the linear fashion that you have written. The market also does not move in a straight line either up or down. If it falls 100 points one day then the options will gain in value. Much also depends on the market sentiment. IF the sentiment is very bearish then the puts will gain value very fast with every fall, even if the strike prices are far away.
When you want to play out these strategies, use options oracle a free software to calculate the payoffs for them. You can get it here:
http://www.samoasky.com/