This describes three of the zero-crossing schemes used in calculating ignition advance numbers in Motronic ECUs. 

ML1.1 .& ML1.2 
The ML1.1 .& ML1.2 is the first of the Motronic designs and the design all later Motronic ECUs are based. The ML1.2 is only capable of positive ignition advance numbers. An ignition advance of zero hex (0h),  gives 0° advance.

 0°  =  0₁₀ =  0h

This reads zero degrees ignition advance.= zero decimal (base 10) = zero hex( base 16) . 

The ML3.1
The ML3.1 is the redesigned ML1.2 and had the ability to go negative. Negative ignition advance capability was added to the design. A  offset of 1e20 decimal (20₁₀) was added to the equation for calculating ignition advance.

 0°  =  20₁₀  =  14h

This reads ... zero degrees ignition advance = 20 decimal = 14 hex..

The M2.1
Porsche skipped the ML4.1 design. Went with  the Bosch M2.1 Motronic on 944S & 944S2 models. It is basically the ML4.1 with added knock capability.  This used the new 60-2 timing wheel system which had become standard. The zero-crossing scheme of the M2.1 changed a little. The offset was changed from 20 to 30 adding more negative range

 0° =  30₁₀  =  1Eh