Above 40 Almost unreadable for most readers.
35-40 Very heavy reading. Just about exceptable if it is technical staff for the technical person - but there are probably a lot of unnecessary abstractions and complex sentences.
30-35 Moderately heavy reading. Acceptable for many Research and Development reports and for an occasional report to senior management on a highly complex subject. But this is too high for normal paperwork.
25-30 Fairly easy reading. About right for most paperwork passing between managers and staff. But still too high a score for sales writing or messages on the notice board.
20-25 Very easy reading. This is the range for personal writing, for hard-hitting selling copy or for notices to be read by all levels in the company.
Below 20 It’s getting childish now! Many readers will probably feel you are writing down to them and you may be oversimplifying your ideas.
The best way to develop an effective style is practice. But if you aim for simplicity, you won’t go wrong.