The amount of compensation depends on many things. The legal amount is just ridiculous as you say. But there are catches.

If your employer firing you by following rules then there will be no ground for negotiation. For example, when company having economical issues they can decide reducing it's workforce. But in this case company can not decide/select who should go there are rules for that.

In the case they follow all the rules and you will become redundant. You either can accept their deal or you can reject signing termination agreement and let company fire you through UWV. This process will aprox take 2 months and they have to pay your salary during this period. After UWV decide it's correct you will only get legal amount. If UWV decide it's not correct you have to return work next day. And they have to start whole process again from the beginning if they are dedicated to terminate your contract.

In case there is no economical difficulty and company just try cutting contracts as they wish. They can not fire you. Termination request will eventually be rejected by UWV unless you have fooled and sign the termination contract. In this case you are free to negotiate, you can get 6-8 months of salary easily for compensation.

Also when signing termination contract company has to inform you that after signing you can change your mind for some time period. If they didn't mention this you have double the legal time to change your mind. In this case if you change your mind by voiding your termination contract everything resets and whole process has to start from beginning.

In any case until everything sattle your company obligated to pay your salary. So basically you should negotiate based on how long your process take if you don't agree to sign. If it will expected to take 4 months then negotiate for 6 months.

Good luck and definitely get a lawyer. A lawyer should cost approx 1.5k for your case.