Talking of ink film thickness on the press, coming down to the plate, looking at historic development, presses could initially only regulate it by regulating the speed of ductor. Later on, possibility to regulate ink thickness zonally was added by manual and later servo driven ink keys.
Fact with modern presses: ink film thickness is regulated with combination of ductor speed and ink key opening.
On the market, there are different printing inks (pigmentation, rheology...) and printing substrates available.
Basic interaction between ink and paper can be observed by ink thickness remaining on the paper. Depending of area covered on the (flat) sheet, corresponding film thickness on (round) rollers, should be provided, to provide even ink film thickness everywhere on paper.
This interaction is not linear. It is defined by so called characteristic curve initially available in CPC 1-04 and later on in CP2000 and Prinect Press Center with much more options (database of characteristic curves, ColorAssistant option for automatic creation/editing of characteristic curves).
Short answer for you: instead of reducing ductor speed, you could close ink zones.
Proper answer: talk to your local Heidelberg on Print Color Management service. One of the activities, provided is also setting correct characteristic curve, which will correspond to your ink/paper/press combination.
Prior to that, take your time and read documentation/presentation from:
Heidelberg - Color and Quality
Janez
PS.: You know, that you can define different paper qualities in the MetaDimension PrintManager Administration/Resources/Printing materials. By entering not only name and dimension (for creeping in Signa and press feeder setup) but also paper type (ISO 12647 style: PT1 glossy coated...). MDS enables transfer of this data from MD to Prinect Signa and Printready/PrepressManager. After you chose in Signa proper material, this information will travel back to MD, which can be effectively used for choosing proper calibration in Calibration manager. It will be written to CIP3 and travel through PPI and PresetLink/InstantGate (not Job memory card) to CP2000/Press Center, which will automatically select characteristic curve for chosen ink/paper combination.