[Al Skierkiewicz]

Phil,
It sounds like you are remanufacturing the transformer core from an ATX power supply into a DC converter power supply. If that is the case, there are too many unknowns at this point for you to accurately make calculations. If you don't have access to a scope then you likely don't have access to other instruments either. An easy way to back into this calculation would be to make a coil that would fit over the core. Measure the inductance with the coil and no core and then measure with the core. The difference would give you a rough idea of the permeability of the core. The ATX power supplies normally run at higher frequencies than what you are designing. Likely 100kHz or higher but the core might still usable at 1.5 kHz. In transformer and inductor design there are other factors involved. The object is to know the action of the core but also allow it to operate in a range of currents that does not saturate the material. This occurs when the current/magnetic field causes the core to reach a point where no additional current will cause a higher magnetic field.
The type of core you describe is a typical "E" design for obvious reasons. The core is made this way so that manufacturing costs are lower. If it is a laminated design (many thin strips of iron) instead of powdered iron, it is better suited for your lower frequency design. (Be aware that a design that stresses a powder core can cause it to crack.) The space between the "E" and the "I" part of the core is a magnetic loss and takes away from the permeability of the core material and as such needs to be included in the calculation.
So now the question is where to start. Check the name on the power supply and see if they have a website. You might be able to find a spec sheet on your power supply that will list the operating frequency. It may also list the components. Then search the net for manufacturers of core material looking for something designed for switching power supplies in your operating frequency range. Manufacturers will usually supply all the data for their products on their websites and may even show design applications that would fit your project. Then it is just a matter of winding the core. Make sure the wire you use will withstand the voltage you expect across the output.
As a word of caution, a bank of capacitors can store a huge amount of energy. At 200 volts, a discharge could be dangerous, even fatal. Be sure to include in your design, bleeder resistors to discharge the caps when the power is removed and never work on the capacitor bank until you are sure that the caps have been discharged.