Philip M. Cronin 53, Interim Rent Control Administrator, defended his rent control guidelines last night before a crowd of 250 people in a meeting more like an unruly ninth grade algebra class than a City Council hearing.

Justifying his definition of the term "fair net income" to mean between 10 and 15 per cent of the market value "We are required by law to insure the landlord some sort of profit. If you invest your money in a savings account of a landlord's property Cronin said, you get back six per cent. Ten to 15 per cent is a reasonable profit rate to expect."

Philip Shaw, an attorney with the Tenants Assistance Project in Cambridge, countered Cronin's statement saying "This guideline does not insure rent control, it insures continual rent increases."

When both men began quoting figures to back their interpretations of the week-old guidelines, Mayor Alfred E. Vellucci interrupted with his gavel. "Now wait a minute. Let's get this arithmetic down so some of us non-lawyers can understand it. Both Phils come up to the board and we'll try to work this out."

After an hour of working through sample determinations of fair net profit for landlords the two interpretations were no nearer agreement. Cronin maintained, "Mr. Shaw's figures are all mixed up." Shaw stated that a fair return for landlords would be nearer eight or nine per cent than 10 to 15.

Cronin said that the controversial guidelines were only temporary and that when a permanent administrator was appointed a hearing would be held to determine permanent guidelines.

City Manager John Corcoran said "I cannot say when the appointment of a permanent administrator will come."

Speaking for the Cambridge Taxpayers Association, attorney William H. Walsh denounced the rent roll back to March 1970 levels, saying "This city will be in bankruptcy in two years if this continues."

Walsh also announced that the Association would begin a tax strike today to protest the law. "We will pay our taxes into an escrow account because we are being assessed twice. Once by taxes and once by rent control," he said.

Thirty members of the Cambridge Tenants Organizing Committee, a radical tenants group that has organized 500 tenants in the city, interrupted the

meeting several times to denounce Cronin.

When finally given time to speak by Mayor Vellucci, Steven Van Kuchen announced, "This is not, as some people seem to think, the late night ego tripping ha-ha show. The problem here is Cronin. Ever since this council passed the rent control law in September he has done everything in his power to stop it and he has got to go."

Cronin sat calmly and folded his hands while Robert Pearlman, another spokesman for the C. T. O. C. hollered "Now you're going to listen to me. This is just like everything else you've tried to do with this law. The government is giving protection to the landlord's profitability and speculation, not relief for the oppressed tenants."