Hello ruud
The first way to set the volume when Kodi starts playing content that I can think of is to set the volume of the TV (or device that feeds your speakers). This is how I do it and have seen it done.
When the initial volume of libespot is set to 100%, librespot/pulseaudio starts playing at the same volume as other contents (audio, video, tv, radio), and the volume of all content can be adjusted in the same manner, usually via the Kodi/CEC or the remote of the TV.
Your question suggests that you are using a specific audio configuration (headless exclusively for Spotify). If so, please elaborate.
Do you play content other than Spotify? If so, how do you adjust the volume?