For offline job I always take a break to do a detailed research about local market, median/max salary, cost of living. Here I often withdraw my candidature due to unsatisfactory Software Engineers compensation in the region. If I decide to proceed, I put myself in highest 20% of distribution. For online it's much easier, I use rates of top Upwork contractors and Toptal (which are already top) and ready to negotiate 10-15% as usually it's fulltime