Unfortunatelly, no one can answer what would happen if you lower the dosages as it is highly variable between different individuals. As a rule, higher dosage = faster climbing Hb but there is no way to know how you respond without testing. Some guys Hb climbs fast even at low dosages while some can take pretty high dosages before anything happens.
Dropping the steroids after your levels have climbed will not restore them to normal levels right away. On average, RBC’s live for about 120 days so in theory, it will take about 4 months before you fully return to base levels. The only way you can lower your levels fast is to get rid of blood.
Also, you write that you just started the cycle last week. If you have only been on for a week, it is highly unlikely that the steroids are the main reason behind your elevated Hb. It is not normal to have Hb at 18.7 naturally (although it certainly is not impossible). Were you dehydrated when the blood was drawn (first thing in the morning or after training)? Have you recently ended a cycle (in the last 4 months)? Do you live at a high altitude? Or do you have any other health issue that you are aware of, like sleep apnea or a lung condition that could explain your elevated levels? If there is no apparent reason, you should see a doctor to find out the cause behind your elevated Hb and rule out disease, especially before you start a steroid cycle. After you rule out any serious issues, if you decide to cycle, you should monitor your Hb closely during the cycle and try to get rid of blood if your Hb levels climb.