Whether you're running a standard distributor type ignition system, or a programmable aftermarket ECU it is essential that the ignition timing is correct for the engine, not only will this ensure the best power is achieved it will also help ensure the engines reliability.
To much ignition advance will quickly destroy pistons on a forced induction motor this cannot be stressed enough !
A slightly advanced ignition on a basic naturally aspirated motor will rarely cause a problem, but on a Turbo engine it can easily cause detonation & lead to complete piston damage & an expensive rebuild.

42628 is the part number for the mini turbo and the MG Metro Turbo dizzy.

Standard ignition timing is 7 degrees BTDC at 1,500 rpm with the vac advance pipe disconnected.

As a basic rule, the further the boost is increased the more the ignition timing should be retarded when on boost to keep the engine free from detonation.
Using a distributor this leaves us with a problem.
On a modified engine using a standard Turbo distributor retarding the timing so the correct advance is seen under boost will most likely leave the ignition timing incorrect at lower revs. This is not a huge issue, unless the ignition timing is very heavily retarded, but is far from ideal if the timing is retarded more than a couple of degrees as power will be lost low down.
The distributor can be modified to give the correct optimised advance curve for the engine but this requires a lot of work altering the sprins & weights internally & some serious rolling road time to determine the engines requirements.
One method of reducing this problem is to limit the amount of total advance the distributor gives. This is done by tacking a blob of weld to the stop of the internal advance mechanism.
The ignition timing can then be set to give a better curve low down & the weld on the stop can be either filed down or built up to adjust the total advance given.

It is also possible to obtain devices which retard the ignition , either when boost is seen or a certain RPM is reached. Though quite crude they are certainly better than nothing.
If using a distributor in conjunction with one of these units more advance can be used low down helping the engine pull better as the timing will be made "safe" when under boost.

The ideal solution to solve all these problems & allow full control of the ignition is to use a modern programmable ECU, this is covered HERE.