How much output needed from MC cart.

It's not that simple. More information is needed.

You have to look at the complete system. Gain is only part of the equation since you have to figure in the efficiency of your speakers.

Add total gain from the electronics (phono stage + line stage + amp) and then calculate if you have enough power to drive your speakers to adequate levels.

As a rough estimate:

Add the gain in dB from all stages of amplification including the amplifier.

Since db = 20 log Vout/Vin, divide that by 20 and take the inverse log of the result.

Multiply the output V of the cartridge times the result to give you output V of the amplifier.

Square this and divide by the impedance of the speaker to get the output power (power = voltage squared/impedance)

If this is enough power to drive your speakers then you should be OK.

Example: You have 69db so far and say you get 20 more from the amp for a total of 89db and you have 8 ohm speakers and we'll use the .8 mV cartridge suggested above.

89dB divided by 20 is 4.45

Inverse log of 4.45 is 28,200.

A cartrige that puts out .8 mV times 28,200 = 22.6V

(22.6V x 22.6V) divided by 8 ohms = 64 watts

Assuming your amp can deliver 64 watts is that enough to drive your speakers?

Jependleton is right on the money for that (lowish) amount of phono preamp gain. I would recommend a van den Hul Frog Gold MkII which has .85 mv output and sounds great when properly loaded at around 500 ohms.

BTW, .85 mv is considered medium output for MC cartridges. High output MC is 1.25 mv +.

You need as a minimum around .8mv of output from the cartridge unless you intend to use a step-up transformer.