Hi there, if you still need, I found a thread that I think might be helpful to you. In that, Spon provides a method for updating what gets displayed within the Tooltip and on the Slider. If you make a Slider with a minimum of 0 and a maximum of 15, you can change the 15 to display as 14.5 in both the Tooltip and Slider. With 15 Grid lines, I think it may come close to what you're looking for. Code is below:
Qualtrics.SurveyEngine.addOnload(function()
{
/*Place your JavaScript here to run when the page loads*/

jQuery("#"+this.questionId+" .numbers li.last").html("14.5");

const QIDtoolTip = this.questionId + "~1~toolTipBox"
const QIDhandle = this.questionId+"~1~handle"

// A bit like cheating, but timer invokes function every millisecond
var timer = setInterval(updateThings, 1);

// desktop or touchscreen
QIDhandle.onmousemove = updateThings;
QIDhandle.ontouchmove = updateThings;
QIDhandle.ontouchend = updateThings;
document.onmousemove = updateThings;
document.ontouchmove = updateThings;
document.ontouchend = updateThings;

// Function called updateThings runs every millisecond.
// Checks if toolTipBox contains 15, then replaces it with 14.5
function updateThings() {
if (document.getElementById(QIDtoolTip).innerHTML.includes("15")) {
document.getElementById(QIDtoolTip).innerHTML = 
document.getElementById(QIDtoolTip).innerHTML.replace("15", "14.5");
}
}
});

Qualtrics.SurveyEngine.addOnReady(function()
{
/*Place your JavaScript here to run when the page is fully displayed*/

});

Qualtrics.SurveyEngine.addOnUnload(function()
{
/*Place your JavaScript here to run when the page is unloaded*/
clearInterval(timer)
});