you can get supplementary benefits ("dagpenge") for up to a maximum of 30 weeks, within the 2-year period with dagpenge.
You will get paid the difference between 160,33 hours and the actual hours you have worked in the month. So if you for example work 100 hours in a month, then you will receive dagpenge for (160,33-100)=60,33 hours.
The Supplementary benerifs is paid like normally dagpenge on a monthly basis. However the supplementary benefits is calculated on a weekly basis. So you get paid for the "missing" hours up to 37 hours per week. In your case (37-20)=17 hours per week.
The level of supplementary benefits is the same as for normal benefits - in 2018 its DKK 18.866 per month for 160,33 hours. So you can calculate how much you can get in Supplementary benefits per hour up to 160,33: 18.866/160,33=117,67 per hour.
Be aware that for being eligible for Supplementary benefits you must meet the normal requirements. This also means that you must be willing to search full-time jobs, while receiving supplementary benefits.
And furthermore your employer has to provide you with a discharge certificate (in danish: "frigørelsesattest") if you are given notice of termination. The discharge certificate means that you can start a new job with more hours at a day's notice.
Anders Weber, www.a-kasser.dk