It depends on what you mean by "discharge xx%". However if you can restate that as "discharge to xx Volts" then the answer may be as simple as using a suitable value resistor in series with a zener diode.
For example, if the capacitor is charged to 300V and you wish to discharge it to 200V, you can use a string of zener diodes which as up to 200V. Let's say you use 4 zener sides of about 50V and rated at 1W.
1W at 50V is 20mA, so the resistor needs to be chosen to limit the current to 20mA at the full capacity voltage. So that's 100V and 20 mA, so the resistor should be 5k at 2W.
Connecting these to the capacitor must be done with care because 300V is easily enough to overcome your skin's resistance giving you a kick you'll never forget. If you get that shock across your chest, it could be the last thing you ever do. (Working with one hand in your pocket is not a silly thing to do)
I would also recommend you connect a multimeter across the capacitor to monitor the voltage across the capacitor.
One sensible option is to connect the discharge circuit across the meter leads at the meter end and then to apply the leads to the capacitor. At this voltage it's safe to rely on the insulation of the probes (assuming they are rated for at least 300V) but if you can connect one and then the other using just one hand you'll be even safer.
Be very sure you have the zener diodes connected around the right way. A simple test is to connect it across a capacitor charged to something less than 100V (to protect the resistor if you made a mistake). If it's connected correctly the calcite should not discharge. Repeat at 200V, and then you should be right to go.
